Various devices are known for forming a seal between a sleeve fixedly mounted on a rotatable shaft and a housing or other structure surrounding the shaft. One type of seal, sometimes referred to as a non-contact circumferential shaft seal, or non-contact gas seal, is effective in controlling leakage. Such seals include one or more seal rings with circumferential inner faces that are spaced a small distance away from the sleeve. Such seals may be formed from compacted and sintered carbon graphite to provide heat and wear resistance, and they are often formed as a plurality of inter-connectable ring segments to facilitate installation around the sleeve. The seal rings are held in place by a suitable retaining device and may include a biasing device, such as a circumferential or garter spring, for holding the seal segments together.
It is desirable to make the gap between the sleeve and seal ring as small as practicable while substantially preventing the sleeve from contacting the seal ring. To this end, it is known to provide cutouts or pads on the radially inner face of the seal ring in order to generate lift relative to the sleeve and maintain a cushion of gas that helps keep the seal ring away from the sleeve. Various arrangements are also known for routing gas around the outer walls of the seal ring to keep the seal ring in a particular axial location and to maintain a radial spacing from the sleeve.